5/7/2023 0 Comments Congenital aural atresia![]() ![]() ![]() (1998) reported that sound localization, as indicated by the percentage of correct identification (within 45°), improved by 53% with binaural listening for three patients with BAHA(s) that were unilaterally or bilaterally fitted. In 1977, a percutaneous bone-anchored hearing aid (BAHA) was developed that avoids most of the drawbacks of conventional BCHAs. One reason for the unilateral application is that the transcranial attenuation (TA) of bone conduction (BC) sound by a BCD is very small (10 dB), so it will stimulate both cochleae to almost the same extent. For a long time, unilateral fitting of BCHAs was commonly applied, even for persons with bilateral CHL caused by microtia, aural atresia, and chronic otitis media. The usefulness of bone conduction devices (BCDs) to assist persons with CHL, such as bone conduction hearing aids (BCHAs), was already pointed out in the early 1950s. Sometimes the term “lateralization” is also used to judge whether the sound appears from the right or the left when presented by a loudspeaker. Related to sound localization, the term “lateralization” is used to describe the apparent location of the sound source within the head, when the stimulus is presented via headphones or bone vibrators. ![]() So, with regard to hearing aids, there are differences in the HRTF depending on the angle at which the sound is presented from the loudspeaker when the device is worn, or where the device microphone is placed on the head. The HRTF changes in the vertical and horizontal planes depending on the angle of incidence of the sound. This filtering can be expressed in “head-related transfer functions (HRTFs)”. Sound localization in the vertical plane is accomplished through filtering by the pinnae and the head itself. The ILD is defined as the difference in the level of a sound at the two ears caused mainly by the head “shadowing” effect for high-frequency components. The ITD is defined as the difference in arrival time between the two ears and is the most important cue to sound localization for low-frequency components. It is well known that sound localization in the horizontal plane is mediated by two cues: interaural time difference (ITD) and interaural level difference (ILD). According to Moore, the term “localization” refers to determining the direction and distance of a sound source. We are surrounded by many different sounds and we can easily know where they are and how far they are from us. Further research on sound localization is necessary to analyze the complicated mechanism of bone conduction, including suprathreshold air conduction with bilateral devices. Moreover, it should be noted that the degree of accuracy in sound localization by bilateral BCDs varied considerably among patients. However, the judgment accuracy was generally lower than that for normal hearing, and the localization errors tended to be larger than for normal hearing. Most studies showed benefits for sound localization or lateralization with bilateral devices. ![]() Recent studies within the past 10 years on sound localization and lateralization by BCDs, middle ear implants, and cartilage conduction hearing aids were discussed. In this review, factors affecting the accuracy of sound localization with bilateral BCDs, middle ear implants, and cartilage conduction hearing aids were classified into four categories: (1) types of device, (2) experimental conditions, (3) participants, and (4) pathways from the stimulus sound to both cochleae. Bilateral bone conduction devices (BCDs), middle ear implants, and cartilage conduction hearing aids have been often applied for patients with conductive hearing loss (CHL) or mixed hearing loss, for example, resulting from bilateral microtia and aural atresia. Sound localization in daily life is one of the important functions of binaural hearing. ![]()
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